Fewer workers than ever are testing positive for illegal drugs at work.

A new study found that the rate of U.S. workers testing positive for cocaine and marijuana has fallen sharply since 1998, but prescription drug abuse may be on the rise, the Wall Street Journal reports. The Quest Diagnostics study found that use of amphetamines, found in drugs like Adderall, among workers has more than doubled since 2002. Vicodin use on the job rose 172% and OxyContin rose 72%. since 2005.

Despite the decrease in marijuana detection at work, more people than ever say they smoke pot. The Department of Health and Human Services found that 7.3% of Americans admitted to smoking marijuana in the last month, up from 5.8% in 2007.

As more states relax restrictions on recreational marijuana use, some employers are facing a conundrum in crafting drug policies. Workers in safety-sensitive industries are prohibited from using marijuana regardless of state law, but employers in other industries are must decide whether smoking pot on the weekend should have professional consequences.